Simon Cowell is half a stone lighter and full of energy after taking a nutritionist’s advice to cut refined sugar, gluten and dairy from his diet.

The 59-year-old X Factor judge felt so sluggish and run down at the start of the year that he consulted a specialist, who tested him for food intolerances.

After taking on board the nutritionist’s advice and changing his eating habits, he has lost some 7lbs and has been ­overheard saying he feels re-energised.

A source close to Cowell said: “Simon was feeling a bit run down, so someone suggested he see a specialist about food tolerances. The specialist revealed he is allergic to a few everyday things which had been making him tired.

“As a result, he has cut out lots from his diet and the impact has been almost immediate. He has loads of energy.”

But Cowell, who has a five-year-old son, Eric, has found his new diet a trial.

The source said: “He’s found it really tough to cut out a lot of things he used to love eating, like sausage rolls.

“But he can’t believe the change it’s had in such a short space of time.

“He’s looking and feeling great at the moment, the best he has in years. He has a real spring in his step, which everyone around him has noticed.”

It is not the first time he has taken action to look and feel better. In his 2012 ­unofficial biography of Cowell, Tom Bower revealed the mogul had used vitamin injections and an IV drip of vitamin B12 to get a boost.

Bower also told how Cowell had “life-enhancing” fresh fruit sent to him by air freight and drank two smoothies a day made from exotic plants. Recently, Cowell has been seen munching carrots and oranges during filming of Britain’s Got Talent.

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He spoke of his dream of looking forever young in 2009, when he got the call to feature on The Simpsons.

He said: “It’s the best thing to get the call. You last forever and are young forever on that show, so I love it.”

Viewers will get the chance to see Cowell’s trim new look on Britain’s Got Talent, with the auditions expected to be shown on ITV next month.